Conventionally, quartz glass has been widely used as a UV transmission material. Quartz glass is produced by using a CVD (chemical vapor deposition) process, a melting process, or other processes. However, these production processes have drawbacks such as extremely high cost and difficulty of upsizing. Particularly, the melting process has the drawback of requiring a very high temperature (1900° C. or above). In recent years, a technique for producing a light source such as a UV region laser or a lamp has been established, and quartz glass has been used for various purposes other than infrared optical communications-use fiber, so that there has been an increasing demand for quartz glass in various ways. Therefore, there is a high demand for a method for producing quartz glass at lower cost.
Further, Patent Document 1 (U.S. Pat. No. 2,106,774) discloses, as a method for mass-producing high silicate glass consisting mainly of silicic acid, a method including the steps of: separating alkaline borosilicate glass into an insoluble phase rich in SiO2 and a soluble phase rich in B2O3 by subjecting the alkaline borosilicate glass to heat treatment; producing porous glass consisting mainly of SiO2, by eluting the soluble phase with acid; and sintering the porous glass. This method is called a Vycor method.
However, the Vycor method causes Fe (iron) ions and water to remain in glass and thus has the problem that UV and IR transmittances cannot be sufficiently increased. For this reason, the Vycor method is not used as a method for producing quartz glass.
As described above, because Vycor glass (glass produced by the conventional Vycor method) contains a very small quantity of Fe ions highly absorptive of short-wavelength UV rays, the Vycor glass is known to have a lower UV transmittance than quartz glass (see Patent Document 2 (Japanese Unexamined Patent Publication No. 205337/1982 (Tokukaisho 57-205337; published on Dec. 16, 1982))). In view of this, in order to increase a UV transmittance of high silicate glass, Patent Document 2 discloses a method for producing high silicate glass, the method including the steps of: subjecting porous glass to further acid treatment with acid containing ethylenediamine tetraacetic acid (EDTA) or a salt thereof; eluting as a water-soluble complex salt a very small quantity of Fe ions contained in the porous glass; and sintering the porous glass.
However, glass produced by the method of Patent Document 2 has an absorption end of approximately 220 nm, and has a much lower UV transmittance than quartz glass, which has an absorption end of 160 nm. Therefore, it cannot be said that the method of Patent Document 2 has the effect of sufficiently removing Fe ions.
The present invention has been made in view of the foregoing problems and has as an object to provide: a method for producing high silicate glass which has a low Fe concentration and can achieve a high UV transmittance while retaining advantages of Vycor glass that mass production at low cost is feasible and that complex formation with various photofunctional ions can be effected; and high silicate glass obtained by the method.